The present invention pertains generally to a metallurgical process for manufacturing an implantable medical device. More particularly, the present invention pertains to a process for producing an improved intravascular medical device, such as a stent, which has improved grain structure in the alloy of the stent which provides improved fatigue and corrosion resistance while maintaining ductility for expansion and strength upon expansion.
Stents are often prescribed to treat arteries that have been blocked or narrowed by atherosclerosis. A stent generally includes a tubular metallic structure that is delivered to the site of the blockage on the balloon of a balloon catheter. With the stent located at the desired position in the artery, the balloon is inflated to expand the diameter of the stent, lodging the stent in the wall of the artery. The balloon can then be deflated and removed from the body. After balloon removal, the stent remains in the artery to maintain a passageway for blood flow through the artery. The wall thickness of the tubular stent is generally made as thin as possible to provide a large passageway for blood flow through the artery.
From a design perspective, the material used to fabricate a stent preferably has good ductility to allow the stent to be expanded by the balloon without fracturing the stent. The stent material preferably also has good strength to ensure that the stent maintains its shape to keep the artery open. Furthermore, during the service life of a stent, the stent is exposed to cyclic stresses from the constant pressure cycling in the artery due to the beating of the heart. Thus, it is important to fabricate the stent from a material having good resistance to fatigue failure. In addition to these mechanical property requirements, the stent materials are preferably chemically stable, corrosion resistant and biocompatible.
Heretofore, stainless steel has been used in the fabrication of stents. In terms of biocompatibility, this material has a substantial history of successful application in invasive medical devices with little or no unacceptable biological response. Table 1 shows the composition range of a particular type of stainless steel (UNS S31673).
A metallurgical process to prepare a stent from type UNS S31673 typically begins with the manufacture of an oversized tube. Next, the oversized tube is annealed at a temperature of approximately 1050 degrees C., leaving the tube in a very soft condition. After annealing, the tube is drawn to the final tube diameter required for the unexpanded stent. This drawing process cold works the steel to a fixed amount, providing the strength required to use the steel in stent applications. This process results in a material having a yield strength in the range of 40-60 ksi and an elongation in the range of 25-40 percent. Alternatively, stents are fabricated from fully annealed stainless steel that has a yield strength between 35-45 ksi and elongation about 50%. The grain size of material manufactured by the above methods is typically between 30-200 grains per 10xe2x88x924 square inch. Although these properties are somewhat satisfactory for relatively thick-walled stents, by improving the mechanical properties (including fatigue resistance) of stent materials, thinner, more reliable stents can be employed.
As previously stated, a material used to make a stent must be formable (i.e., have sufficient ductility and weldability to be formed into the appropriate final stent shape), and yet needs to provide good mechanical properties in the finished stent to hold the lumen open. Stainless steel and alloys, such as the radiopaque alloys disclosed in U.S. patent application Ser. No. 10/112,391, filed Mar. 28, 2002, entitled xe2x80x9cPlatinum Enhanced Alloy and Intravascular or Implantable Medical Devices Manufactured Therefromxe2x80x9d (the disclosure of which is incorporated herein by reference), are readily formable, can be strengthened by work hardening, and exhibit good mechanical properties in finished stents. Furthermore, these alloys are readily weldable due to low carbon content. As for biocompatibility, the alloys have a successful history in invasive medical device applications. However, it would be beneficial to utilize improved manufacturing techniques that maintain the above properties while also improving corrosion resistance and reducing the likelihood of fatigue fracture, especially in thin-walled stents.
The present invention is directed to a method for producing a metallic medical device having improved mechanical properties. The process is particularly useful for manufacture of implantable medical devices and/or intravascular medical devices. The alloy used is preferably stainless steel or an enhanced radiopaque alloy as disclosed in U.S. patent application Ser. No. 10/112,391, filed Mar. 28, 2002, entitled xe2x80x9cPlatinum Enhanced Alloy and Intravascular or Implantable Medical Devices Manufactured Therefrom.xe2x80x9d A preferred medical device of the present invention includes a stent which is a generally tubular structure having an exterior surface defined by a plurality of interconnected struts having interstitial spaces therebetween. The generally tubular structure is expandable from a first position, wherein the stent is sized for intravascular insertion, to a second position, wherein at least a portion of the exterior surface of the stent contacts the vessel wall. The expanding of the stent is accommodated by flexing and bending of the interconnected struts throughout the generally tubular structure.
The present invention includes a preferred metallurgical process for producing an implantable medical device, such as a stent, in a condition wherein the alloy of the stent has improved mechanical properties. For the present invention, the starting material is preferably a stainless steel, such as UNS S31673. The starting material can also include a platinum enhanced alloy as previously cited. In the starting material, the levels of carbon and other interstitial elements such as nitrogen and oxygen are controlled below a predetermined amount to reduce the occurrence of grain boundary precipitates during the metallurgical process. In preferred embodiments, carbon and oxygen are controlled to below 0.03 and 0.02 weight percent, respectively.
In accordance with preferred embodiments of the present invention, the starting material is cold worked to produce a material having a high dislocation density and a yield strength that is above approximately 125 ksi. Generally, to obtain this cold worked state, the starting material is deformed to a strain of approximately 30-55 percent. In manufacturing a stent, this step is accomplished by beginning with an oversized tube which is first drawn to about the final diameter of the unexpanded stent. The strain that occurs during tube drawing is generally sufficient to cold work the material. Furthermore, because of the thin-walled nature of the tube, uniform strains are imparted throughout the tube during the drawing process, leading to a stent having uniform properties.
Next, the cold-worked material is heat treated at a temperature of approximately 0.5TM, where TM is the absolute melting temperature of the alloy. In preferred embodiments, heat treatment is conducted between 0.4TM and 0.6TM. During this heat treatment, self diffusion is enabled and vacancies become mobile. This allows dislocations to rearrange and form cells of dislocations (i.e., sub-grains), as well as re-crystallization of the material grain structure. Because the levels of carbon and the other interstitial elements are controlled below a predetermined amount, unwanted grain boundary precipitates are reduced (e.g., minimized) during the heat treatment. Further, because of the thin-walled nature of the stent, heat treatment can be accomplished quickly. For a stent of wall thickness 0.005 inch, heat treatment is completed in about 30 to 40 minutes.
Upon cooling, a material is obtained having a yield strength between about 40-65 ksi and an elongation exceeding 40 percent. The resultant material also has good corrosion resistance due to the reduction of grain boundary precipitates. Furthermore, the sub-grains and re-crystallized grain structure that are established during the heat treatment provide improved fatigue resistance. Unlike prior manufacturing processes, cold working after heat treatment to improve the mechanical properties of the stent is not necessary.